Long-term effects of liquid swine manure on soil organic carbon and Cu/Zn levels in soil and maize

Swine feed is often supplemented with Cu and Zn, but > 90% of the ingested metal is excreted, ultimately accumulating in soil treated with liquid swine manure (LSM) to meet nitrogen (N) requirements. A maize field experiment was conducted between 2002 and 2015 in the Ebro Valley (north-east Spain) to determine the effects of long-term LSM application at two rates [30 and 50 m3 ha−1, equivalent to ~ 220 (218) and ~ 390 (389) kg N ha−1] compared to 0 and 300 kg N ha−1 mineral fertilizer. The highest Cu concentration in plants (5.03 mg kg−1) was observed following the 300 kg N ha−1 treatment. The treatments did not affect plant Zn levels (~ 21 mg kg−1). All whole-plant and grain metal concentrations were lower than threshold values for animal and human ingestion (30 mg Cu kg−1 and 500–1300 mg Zn kg−1). After 12 years, the soil organic carbon (SOC) of 13 g kg−1 in the first year (2002) increased by 19% and 26% at the low and high LSM rates, respectively. Mineral fertilization (300 kg N ha−1) achieved a 22% increase in SOC and also the 0 kg N ha−1 control treatment caused the SOC to rise by 11%. LSM applications do not seem to have adverse effects on maize yields after 12 years of applications.